Side rail and method for producing a hot-formed and press-hardened side rail

ABSTRACT

A side rail and to a method for producing a side rail are disclosed. The side rail has a region of a first type and a region of a second type which have mutually different strengths. A transition region having a width of less than 50 mm is formed between the two regions. The side rail has in the region of the first type a bainitic structure and in the region of the second type a martensitic structure.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of German Patent Application,Serial No. 10 2010 048 209.9, filed Oct. 15, 2010, and European PatentApplication Serial No. 11 155 717.9, filed Feb. 23, 2011, pursuant to 35U.S.C. 119(a)-(d), the content of which is incorporated herein byreference in its entirety as if fully set forth herein.

This is one of two applications both filed on the same day. Bothapplications deal with related inventions. They are commonly owned andhave the same inventive entity. Both applications are unique, butincorporate the other by reference. Accordingly, the following U.S.patent application is hereby expressly incorporated by reference:“AUTOMOBILE COLUMN AND METHOD FOR PRODUCING A HOT-FORMED ANDPRESS-HARDENED AUTOMOBILE COLUMN”.

BACKGROUND OF THE INVENTION

The present invention relates to a side rail, produced by hot-formingand press hardening. The present invention also relates to a method forproducing a side rail by hot-forming and press hardening.

The following discussion of related art is provided to assist the readerin understanding the advantages of the invention, and is not to beconstrued as an admission that this related art is prior art to thisinvention.

The requirements profile for vehicle safety increases in the automotiveindustry due to regulatory and manufacturer-specific guidelines. At thesame time, the automobile manufacturers strive to reduce the weight ofthe automobile bodies in order to minimize fuel consumption and CO₂emission. This creates a divergence between low weight and high bendingand torsion strength and high crash safety.

According to one approach, for example light-metal materials, inparticular aluminum alloys, or bodies in hybrid construction, forexample made of metallic alloys and fiber composite material orplastics, can be used. However, the aforementioned approaches are bothassociated with high material costs, which in turn increases the vehicleproduction costs of models produced in large quantities.

However, a metallic alloy, in particular steel, still remains thepreferred material for constructing the body, in particular the rawbody. Due to consequent improvements, steel is still viewed as ahigh-tech material which due to different processing approachesrepresents a good compromise between favorable manufacturability,excellent crash safety and long service life.

Heat-treatment is according to the state-of-the-art typically performedin a temperature range between 320° C. and 400° C. and hardly changesthe material properties and the strength values adjusted in thehot-forming and trans-hardening process. At the same time, however, theductility of the material is increased so as to allow superior foldformation in a crash.

However, the additional heat-posttreatment once more increases theproduction costs due to significantly higher tooling costs up to thestart of the series production.

It would therefore be desirable and advantageous to obviate prior artshortcomings and to provide an side rail and a method for itsmanufacture, which has lower manufacturing costs compared to thestate-of-the-art, while simultaneously allowing precise adjustment ofmaterial properties within the side rail.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a side rail with atleast two regions of different strength is produced by hot-forming andpress-hardening, wherein a region of a first type has afterpress-hardening a substantially bainitic structure and a region of asecond type has after press-hardening a substantially martensiticstructure, and a transition region between the region of the first typeand the region of second type being smaller than 80 mm.

According to one advantageous feature of the present invention, thetransition region is smaller than 50 mm, preferably smaller than 30 mmand still more preferably smaller than 20 mm. Because the transitionregion is very small, the component can within the context of theinvention be specifically adjusted in a single production step, namelythe production process itself, so that the required crash properties canbe reliably implemented with the current manufacturing tolerances, whilesimultaneously having improved manufacturability.

According to another advantageous feature of the present invention, aside rail with advantageous material properties can be attained inpreviously defined regions of the first and second type by a reliableprocess and a specific design. During the hot-forming andpress-hardening of a metal plate or of a preform or semi finishedproduct made of high-strength hardenable steel, regions of the firsttype are intermediately cooled, so that regions of the first type andregions of the second type with different strengths, hardness andductility properties can be intentionally adjusted. A material structurehaving a more ductile tendency is produced in the regions of the firsttype, as compared to the regions of the second type. The transitionregion between both regions tends to have clearly defined edges. Thissignificantly relaxes or even entirely eliminates production tolerances.Regions with ductile material properties are produced by thesubstantially bainitic structure in the region of the first type.

An intentional deformation is favored in the regions of the first type,so that folds or a compressions can be formed in a crash, withoutcausing cracks or detachment. The energy absorption capacity of the siderail according to the invention is hereby increased while retaining itshigh stiffness. A high degree of energy is then absorbed in anautomobile equipped with the side rail according to the invention byconverting kinetic energy from the impact into deformation energy, whileretaining the high stiffness of the rest of the automobile body.

Moreover, the side rail may be used as engine support or may be employedin the area of the luggage compartment, where a higher energy absorptioncapacity may be required than in the region of the passenger compartmentitself. A sequence of regions of the first type and regions of thesecond type may be produced along the length of the side rail accordingto the invention in the direction of the vehicle by specificallycreating regions of the first type and regions of the second type. Theside rail can the advantageously collapse in a crash like an accordion.

The side rail according to the invention may be arranged, for example,in an automobile body transversely at the front and/or rear side tointentionally absorb an impacting object or another automobile. The siderail should absorb the body colliding with the automobile or thestationary body hit by the automobile such that the side rail isminimally deformed for dissipating the energy and a deliberate intrusionof a body into the automobile itself is prevented. With the side railaccording to the invention, the overall energy absorption capability isincreased, which in turn increases the overall energy absorptioncapacity of the automobile body, while at the same time providing a highstiffness. In addition, material savings can be achieved in the regionsof higher ductility because of, for example, thinner wall thicknesses,which in turn reduce the overall weight of the automobile body. A highdegree of energy is then absorbed in an automobile equipped with theside rail according to the invention in that kinetic energy of theimpact is converted into deformation energy, while the stiffness of thepassenger compartment and hence of the body remains high or is evenincreased.

According to another advantageous feature of the present invention, aside rail according to the invention may also prevent unintentionalbuckling in regions which are intentionally formed as regions of thesecond type after hard-forming and press-hardening according to theinvention. The high hardness of the regions of the second type thereforeprevents an undesirable deformation in certain regions.

Weakening of the material caused by vibrations under high permanent loadand/or vibrations in conjunction with a high stiffness is alsoprevented. The remaining components of the side rail, i.e. the regionsof the second type, have a substantially martensitic structure withparticularly high strength values, so that the minimally requiredstrength and crash characteristic of the component is adequatelyattained.

The side rail produced according to the invention can thus be producedmore cost-effectively compared to components produced with conventionalproduction methods, because only a single reforming and press-hardeningprocess is required for adjusting the most important required propertiesof the components. The adjustment by way of a substantially martensiticstructure and an intermediate structure which is substantially definedby a bainitic structure, allows a particularly error-free, specificadjustment of the required material properties in clearly definedregions of the component.

According to another advantageous feature of the present invention, theregion of the second type, which has a martensitic structure as themajor component of the structure, includes other structures inconcentrations of more than 50%, in particular more of than 80%,preferably of more than 90%, and particularly preferred situations ofmore than 95%.

The high torsional stiffness and bending stiffness due to themartensitic structure guarantees the elevated hardness of the side railaccording to the invention, which preserves the integrity of the bodyand of passenger compartment as much as possible and thus protects thevehicle occupants.

According to another advantageous feature of the present invention,bainite may be present as an additional structure component in theregion of the second type.

According to another advantageous feature of the present invention, theregion of the first type may have as the primary structure componentbainite, wherein additional structure components with less than 50%,preferably less than 30%, and in particular less than 15% may bepresent. For example, a mixed structure of bainite, with ferrite and/orperlite may be present. Optionally, within the context of the invention,martensite may also be present as a component of the structure in theregion of the first type.

According to another advantageous feature of the present invention, theregion of the first type may be at least partially enclosed by theregion of the second type; preferably, the region of the first type maybe completely enclosed by the region of the second type. The region ofthe first type is preferably completely enclosed by the region of thesecond type in the region of the attachment points for, for example,crash boxes. Due to the particularly small transition region accordingto the invention, the stiffness in the direction of the componentremains unchanged, so that essentially no weakened location, for examplein form of an undesired rated breakpoint, is produced. The region of thefirst type is also ductile so as to prevent crack formation as much aspossible. The ductility of the region of the first type also largelyprevents components attached to the side rail or other couplescomponents from being torn off, for example in an offset crash.

According to another advantageous feature of the present invention, theregion of the first type may be spot-shaped, preferably with a diameterof less than 40 mm, in particular less of than 20 mm and particularlypreferred of less than 10 mm.

According to another advantageous feature of the present invention, apassage may be produced in the region of the first type. This means thepassage may be formed simultaneously during the reforming process and/orpress-hardening process; in one embodiment, the passage can also becreated after the end of the press-hardening process. Due to the higherductility, tool wear of a punching or stamping tool is reduced, or thepassage can only be produced by this process without crack formation.

According to another advantageous feature of the present invention,marginal regions, in particular recesses and flanges, may be formed asregions of the first type, wherein cracks originating from the edge canbe effectively prevented. Also regions subjected to mechanicalprocessing after press-hardening, such as re-orientations, canadvantageously be implemented as regions of the first type.

According to another advantageous feature of the present invention, theregion of the first type may also be provided as a region for producingcutting edges. This provides an initial material characteristic which isgentle on the cutting or separation tool to advantageously allow coldcutting after hot-forming and press hardening, for example with simplecutting and/or separation methods. Further machining of the component,for example by cutting, is here particularly gentle, precise andcost-effective while maintaining the required tight tolerances. Inparticular, the need for an expensive laser cutting of the otherwisehard edge of the component can be eliminated. To this end, acircumferential, narrow region of the second type can advantageously beformed proximate to the edge contour. The risk of a delayed formation ofcracks, caused by local stress in the hard structure, is at the sametime significantly reduced.

According to another advantageous feature of the present invention, theregion of the first type may have a stretchability A50 between 10 and30%, preferably between 14 and 20%. This ensures sufficiently highstrength, with simultaneously adequate ductility, thereby preventing theformation of cracks and hence individual structural automobilecomponents to be torn off in a crash.

According to another advantageous feature of the present invention, theregion of the first type may have a tensile strength between 500 and1000 N/mm², preferably between 550 and 800 N/mm². The region of thefirst type may have an elongation limit between 200 and 800 N/mm²,preferably between 250 and 600 N/mm², particularly preferred between 250and 500 N/mm², and even more preferred between 300 and 500 N/mm².

Between the region of the second type and the region of the first type,the elongation limit and/or the tensile strength may be formed with adecreasing or increasing gradient of more than 100 N/mm², preferablymore than 200 N/mm², and in particular more than 400 N/mm² per 10 mm.This means that the elongation limit and/or the tensile strength in theregion of the first type increase by more than 100 N/mm² per 10 mm inthe direction of the region of the second type.

According to another advantageous feature of the present invention, theregion of the second type may have a strength of more than 1000 N/mm²,in particular of more than 1200 N/mm², and preferably of more than 1400N/mm².

According to another aspect of the invention, a method for producing ahot-formed and press-hardened side rail, wherein the side rail has atleast two regions of different strength, includes the following methodsteps:

-   -   providing a hardenable metal plate or semi-finished product and        heating the hardenable metal plate or semi-finished product to        at least an austenizing temperature,    -   intermediately cooling a region of a first type of the metal        plate or semi-finished product with a cooldown speed selected to        be greater than a lower critical cooldown speed of a material of        the metal plate or semi-finished product, and    -   hot-forming and press-hardening the metal plate or semi-finished        product in a press-hardening tool to form the side rail.

With the method according to the invention, an intermediate stagestructure may be adjusted under time control and/or temperature control.The intermediate stage structure may be adjusted, in particular, in theregion of the first type of the metal plate by intermediate cooling. Thecooldown speed of the intermediate cooling may be selected within thecontext of the invention so as to be above the lower critical cooldownspeed of the bainite formation of the material of the metal plate. Thecooldown speed may also be greater than the lower critical cooldownspeed of the bainite formation. In particular, those regions are cooledwhich are designed to be soft after press-hardening, i.e., they have ahigher ductility.

According to one advantageous feature of the present invention, thecomponent may also be preformed to a semi-finished product while cold.The component is then at least partially preformed from a hardenablemetal plate. Preferably, preforming matches at least 80% of the finalshape of the component. Following the cold preforming process, which canbe carried out, for example, at room temperature, a heating step to atleast the austenizing temperature, i.e. to above the AC3 temperature, isperformed. Thereafter, a region of the first type is at least partiallyintermediately cooled, followed by additional steps of the methodaccording to the invention.

The cooldown process of the intermediate cooling is performed after thehardenable metal plate is heated to the austenizing temperature, but mayalso be performed within the context of the invention before or duringthe hot-forming and press-hardening process. In particular, if thecooldown process of the intermediate cooling is performed duringpress-hardening, suitable means are provided in the pressing toolcapable of performing a corresponding cooldown as well as correspondingcooldown speeds.

If the intermediate cooling takes place before hot-forming andpress-hardening, then this may be associated with a production line withcorresponding intermediate transfers of the metal plate that was heatedabove the austenizing temperature.

The cooldown itself may be performed, for example, by free or forcedconvection, with cooling rollers, two-sided or one-sided annealingplates with an insulated abutment or by applying cooling media, such aswater, or with other suitable cooling devices. The cooldown can herebybe performed in a fixedly installed intermediate station as well as in acooling unit which moves commensurate with the production cycle.Preferably, a cooldown speed for the intermediate cooling is between 200Kelvin per second and 5 Kelvin per second. Particularly preferred is acooldown speed of 50 Kelvin per second. The cooldown is herebypreferably performed immediately after removal from the furnace. In thisway, strength values between 550 and 900 MPa are adjusted in the firstregions. Preferably, strength values of substantially 700 MPa areadjusted.

According to another advantageous feature of the present invention, aregion of the second type may be held above the austenizing temperature,wherein the region of the second type may be any region of the metalplate that is not taken up by the region of the first type. This meansthat after the metal plate is heated to at least in the austenizingtemperature, a corresponding temperature above the austenizingtemperature is maintained. This may be done actively by using externalheat sources, or passively by employing suitable insulation. Atemperature above the temperature AC1 may also be maintained. Although acertain loss in strength may occur compared to forming from AC3, this isnoncritical in most situations.

When employing external heat sources, the temperature may be held in theregion of the second type, in particular with infrared lamps, heatingcoils, pore burners, insulation plates or similar external heat sources.Within the context of the invention, a temperature significantly abovethe austenizing temperature may be selected, wherein the time after theheat-up to above the austenizing temperature has ended to the start ofthe press-hardening process and the accompanying cooldown are matched toone another such that the region of the second type is at the start ofthe press-hardening process still at a temperature which is at leastabove the austenizing temperature.

According to one advantageous feature of the present invention, thecooldown speed during intermediate cooling of the region of the firsttype may be selected so that a bainitic structure is obtained;preferably, the material is cooled down to a temperature between 700 and400° C., preferably 650 to 450° C., and in particular 650 to 500° C.With cooldown speeds that are greater than the lower critical cooldownspeed of the respective employed material, but stop above themartensitic start temperature, the so-called bainite formation occursduring isothermal holding of the cooldown temperature, also known asintermediate structure or as intermediate stage.

Unlike with conventional methods, where perlite or ferrite is formed,with perlite being formed mainly directly from the austenite bydiffusion, the diffusion of carbon in the austenite is significantlyhindered in the intermediate stage of the bainite as a result of themore rapid cooldown. Small austenite regions, mostly originating atgrain boundaries, are transformed during bainite formation into adistorted alpha lattice. Because the diffusion velocity in the alphalattice is significantly greater than in the gamma lattice, smallcementite grains precipitate in these alpha mixed crystals which areoversaturated with carbon, which become finer with faster cooldown. Thisproduces a substantially needle-like bainitic structure. This alsoproduces a grainy structure of the carbides caused by the increasinghardness which increases with the grain fineness. A further differenceis made in the bainite structure between an upper intermediate stage, inwhich the carbides are combined for increased incursion, and a lowerintermediate stage, in which the carbides are very finely distributed.

According to one advantageous feature of the method of the presentinvention, the region of the first type may be maintained at thecooldown temperature of the intermediate cooling for a predeterminedtime; preferably, the temperature may be held substantially isothermal.With this embodiment, the respective required or desired strength valuesof the bainitic intermediate structure can be adjusted exactly as afunction of time. The intermediate cooling in this embodiment takesplace substantially to a temperature where the material structure in theregion of the first type has been transformed into austenite, or occursdirectly into the intermediate structure. From this cooling temperature,the material structure is further transformed by isothermal holding fora specified time. The material is then transformed from an austeniticstructure to a bainitic structure. If the material is cooled directlyinto the intermediate stage by selecting the cooldown speed, then amixed structure between austenite and bainite are already adjusted. Byholding at the cooldown temperature, holding is performed for apredetermined time in a purely bainitic structural transformation range.The longer the region of the first type is held at the temperature, thegreater becomes the bainitic component of the structure.

According to another advantageous feature of the present invention, theintermediate structure range cooled to the cooldown temperature may befurther quenched from the bainitic structural transformation stage inthe press-hardening tool itself, so that a mixed structure of martensiteand bainite is adjusted in the region of the first type. By quenchingthe region of the first type, where the structure has an intermediatestage, the residual austenite fractions are transformed to martensitefractions during press-hardening. As a result, a martensite-bainitemixed structure is produced in the regions of the first type. Thefractions of the bainite in relation to martensite depend again from theduration during which the first region is held in the intermediatestage, before the press-hardening process begins.

According to another advantageous feature of the present invention, theregion of the first type may be held isothermally during a certain timeinterval so as to transform the region of the first type is completelyinto bainite. This produces a material structure with a higher strengthcompared to a ferritic-perlitic structure. In particular, a perliticstructure is hereby intentionally avoided, which would reduce theductility.

According to another advantageous feature of the present invention, thecooldown speed during intermediate cooling may be selected to be above acritical cooldown speeds of the employed material. In this way, anaustenitic region can be selectively adjusted which is thereafter held,preferably isothermally, during a predetermined time at a temperaturelevel, so that the structural transformation is specifically adjusted tobe bainitic during the holding time. Depending on the employed holdingtime, a partially bainitic-austenitic structure or an exclusivelybainitic structure can be adjusted. If a bainitic-austenitic structureis adjusted, this structure is transformed to a bainitic-martensiticstructure in the subsequent press-hardening process.

Within the context of the invention, holding is to be understood asmaintaining a substantially identical temperature below the ferrite andperlite temperature, but above a martensite start temperature, i.e.substantially below 700° C., in particular below 600° C., particularlypreferred below 550° C. For example, when isothermally holding for alonger time, the temperature may decrease from 500 to 400° C., whichhowever is still considered within the context of the invention to besubstantially isothermal. Particularly preferred, the region of thefirst type is held isothermally during a time interval from 1 second to80 seconds. Particularly preferred, the holding time is 15 seconds.However, these values are to be selected depending on the employedmaterial alloy.

According to another advantageous feature of the method of the presentinvention, the intermediate cooling of the region of the first type maybe performed in the press-hardening tool, for example with coolingplates arranged in the press-hardening tool. This reduces the cycletimes and also the production costs. In particular, an automobilecomponent having a region of different strength is produced with onlytwo tool steps. Initially, heat-up is performed in a furnace system,followed by a combination of intermediate cooling and hot-forming andpress-hardening using only a single tool.

A cooldown speed of at least 25 Kelvin per second may be selected as thecooldown speed in the actual press-hardening process. In anotherembodiment, the cooldown speed may be selected to be higher than 27Kelvin per second. However, higher cooldown speeds may be selected forthe actual press-hardening process. In particular, the press-hardeningprocess may then be performed both in the region of the first type andin the region of the second type at the same cooldown speed depending onthe local temperature gradient between press-hardening tool and theworkpiece. Due to the different temperatures at the start of thepress-hardly process in both regions, the cooldown speed may slightlydiverge from the region of the first type to the region of the secondtype.

In one embodiment, a hardenable steel categorized as micro-alloyedheat-treated steel is used with the method according to the invention.This steel includes in particular the following alloy element in massweight percent fractions:

carbon (C) 0.19 to 0.25 silicon (Si) 0.15 to 0.30 manganese (Mn) 1.10 to1.40 phosphorus (P) 0 to 0.025 sulfur (S) 0 to 0.015 chromium (Cr) 0 to0.35 molybdenum (Mo) 0 to 0.35 titanium (Ti) 0.020 to 0.050 boron (B)0.002 to 0.005 aluminum (Al) 0.02 to 0.06.

According to one advantageous feature of the present invention, theintermediate cooling of the regions of the first type may be performedwith a tool having integrated cooling plates. The cooling plates mayhere have an intrinsic temperature of up to 600° C., which is still lessbelow the AC3 temperature of more than 900° C. The region of the firsttype can be cooled down with these cooling plates and then, if desired,held isothermally for a certain time. For example, such cooling platescan be brought to the respective required temperature with electricalheater cartridges or by backside burner heating or with thermal oils.

According to one advantageous feature of the present invention, theintermediate cooling may also be performed with substantially coldcooling plates. The cooling plates then have a temperature significantlybelow 400° C., preferably between −100° C. and +100° C., particularlypreferred between −10° C. and +25° C. However, an isothermal holdingtime can only be performed with cold cooling plates a limited way. Inone embodiment, both versions of cooling plates may be integrated, forexample, in a hot-forming tool and pressing tool, so that the entireprocess following the actual furnace heating is performed in only asingle tool. Within the context of the invention, the cooling plates forperforming the intermediate cooling may also be housed in a separatetool, so that the process takes place from a heat-up furnace viaintermediate cooling to the actual hot-forming in press-hardening tool.This embodiment has the advantage that the separate tool can be designedsubstantially as a flat tool with substantially flat heating and/orcooling plates.

BRIEF DESCRIPTION OF THE DRAWING

Other features and advantages of the present invention will be morereadily apparent upon reading the following description of currentlypreferred exemplified embodiments of the invention with reference to theaccompanying drawing, in which:

FIG. 1 shows a detail area of a side rail according to the inventionwith a region of a first type, a transition region and a region of asecond type;

FIG. 2 shows a side rail according to the invention;

FIG. 3 shows a time-temperature diagram for carrying out a processaccording to the invention; and

FIG. 4 shows a side rail assembly according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Throughout all the figures, same or corresponding elements may generallybe indicated by same reference numerals. These depicted embodiments areto be understood as illustrative of the invention and not as limiting inany way. It should also be understood that the figures are notnecessarily to scale and that the embodiments are sometimes illustratedby graphic symbols, phantom lines, diagrammatic representations andfragmentary views. In certain instances, details which are not necessaryfor an understanding of the present invention or which render otherdetails difficult to perceive may have been omitted.

Turning now to the drawing, and in particular to FIG. 1, there is showna detail of a side rail 1. As can be seen, a region of the second type 3according to the invention is formed in a region of the first type 2. Atransition region 4 is arranged between the region of the first type 2and the region of the second type 3. A material structure having atendency to be ductile is produced in the region of the first type 2,whereas a hard material structure is produced in the region of thesecond type 3. Within the context of the present invention, thetransition region 4 has essentially a width a which is quite smallcompared to the region of the first type 2.

FIG. 2 shows a side rail 1. The side rail 1 has beads 5, openings 6 andrecesses 7. The side rail 1 according to the invention also has joiningflanges 8 disposed in its marginal regions. The beads 5, openings 6,recesses 7 and joining flanges 8 are each implemented as regions of thefirst type, depending on the requirements, whereas the remaining regionof the side rail 1 is implemented as a region of the second type.

FIG. 3 shows a time-temperature diagram of an exemplary steel, withoutlimiting the field of the present invention. Several structures areindicated which are obtained in the material at various cooldown speedsas a function of temperature. The lower part of the FIG. shows themartensite formation. Above, in the center region of the FIG., thebainite formation is shown, and there above the perlite and/or ferriteformation.

In the illustrated exemplary embodiment, three different curves for thedifferent cooldown processes are shown. Curve K1 shows the course of thetemperature for a first region according to the invention, wherein thisregion is first heated to a temperature above the AC3 temperature. Fromthis temperature, the material is cooled down to an intermediatetemperature of about 520° C. with a cooldown speed which in this case isgreater than the upper critical cooldown speed oK for the bainiteformation of the illustrated material. When the cooldown temperature ofthe intermediate cooling of about 520° C. is reached, the first regionis held substantially isothermally at a temperature for the time ti. Thetemperature thereby decreases from about 520° C. to about 480° C. due toheat loss in form of, for example, heat radiation, convection or heatconduction. An austenitic structure is produced at the time Z1 of theintermediate cooling, and a bainitic-austenitic mixed structure isproduced at the time P1, corresponding to the start of press-hardeningin the first embodiment.

In the first embodiment, quenching thereafter occurs in thepress-hardening process from the time P1, such that thebainitic-austenitic mixed structure in the first region is transformedto a bainitic-martensitic mixed structure. In parallel, the secondregion according to the invention is quenched from a temperature aboveAC3 by press-hardening, producing a martensitic structure directly froman austenitic structure; however, this is not illustrated in detail forsake of clarity.

The second embodiment of the method according to the invention isillustrated with the cooldown sequence according to curve 2 of the firstregion. The cooldown sequence of the curve 2 is similar to the cooldownsequence of the curve K1, wherein the cooldown temperature is held for alonger time from a time Z2 (equal to Z1), so that the press-hardeningprocess starts at a time P2. The time interval t2 is therefore greaterthan t1. The structure in the first region is completely transformed tobainite at the time P2 and therefore does not undergo any furtherstructural transformation after the time P2 due to the cooldown speed.

In a third embodiment according to the present invention, a cooldownspeed from a temperature above the AC3 temperature according to curve 3is selected, so that a transformation occurs directly into the bainiticintermediate structure during the cooldown process of the intermediatecooling. In the first region, an austenitic-bainitic intermediatestructure was adjusted, so that when the press-hardening process startsat the time P3, this bainitic-austenitic mixed structure in the firstregion is transformed to a bainitic-martensitic mixed structure. In theembodiments according to curves 2 and 3, the second region which washeld above the AC3 temperature during the intermediate cooling, is inboth cases transformed from the austenitic region directly to martensiteby the cooldown during the press-hardening process. In the embodimentaccording to curve 3, the temperature is selected according to theinvention to be always greater than the lower critical cooldown speedsuK of the corresponding employed material.

FIG. 4 shows a side rail assembly 9 formed of a side rail 1 and aheat-treated component 10. The side rail 1 is here formed in the centerregion as a region of the second type and in an outer region as theregion of the first type. The side rail 1 and the component are coupledwith one another at their corresponding lateral regions by joiningflanges 8. The joining flanges 8 themselves are here formed as regionsof the first type with a rather ductile material characteristic. In theevent of a deformation, for example in a crash, a basic stiffness isprovided by the side rail 1 itself. Detachment is prevented by therather ductile material characteristic. Both components are connectedwith each other at the coupling locations 11.

While the invention has been illustrated and described in connectionwith currently preferred embodiments shown and described in detail, itis not intended to be limited to the details shown since variousmodifications and structural changes may be made without departing inany way from the spirit and scope of the present invention. Theembodiments were chosen and described in order to explain the principlesof the invention and practical application to thereby enable a personskilled in the art to best utilize the invention and various embodimentswith various modifications as are suited to the particular usecontemplated.

1. A side rail comprising: at least two regions of different strengthproduced by hot-forming and press-hardening, wherein a region of a firsttype has after press-hardening a substantially bainitic structure and aregion of a second type has after press-hardening a substantiallymartensitic structure, and a transition region between the region of thefirst type and the region of second type being smaller than 80 mm. 2.The side rail of claim 1, wherein the transition region is smaller than50 mm.
 3. The side rail of claim 1, wherein the transition region issmaller than 30 mm.
 4. The side rail of claim 1, wherein the transitionregion is smaller than 20 mm.
 5. The side rail of claim 1, wherein thesubstantially martensitic structure of the region of the second typecomprises additional structure components in a concentration of lessthan 50%.
 6. The side rail of claim 1, wherein the substantiallymartensitic structure of the region of the second type comprisesadditional structure components in a concentration of less than 30%. 7.The side rail of claim 1, wherein the substantially martensiticstructure of the region of the second type comprises additionalstructure components in a concentration of less than 15%.
 8. The siderail of claim 5, wherein the additional structure component comprisesbainite.
 9. The side rail of claim 1, wherein the substantially bainiticstructure of the region of the first type comprises additional structurecomponents in a concentration of less than 50%.
 10. The side rail ofclaim 1, wherein the substantially bainitic structure of the region ofthe first type comprises additional structure components in aconcentration of less than 30%.
 11. The side rail of claim 1, whereinthe substantially bainitic structure of the region of the first typecomprises additional structure components in a concentration of lessthan 15%.
 12. The side rail of claim 1, wherein the region of the firsttype is at least partially enclosed by the region of the second type.13. The side rail of claim 12, wherein the region of the first type iscompletely enclosed by the region of the second type.
 14. The side railof claim 1, wherein the region of the first type is spot-shaped with adiameter of less than 40 mm.
 15. The side rail of claim 1, wherein theregion of the first type is spot-shaped with a diameter of less than 20mm.
 16. The side rail of claim 1, wherein the region of the first typeis spot-shaped with a diameter of less than 10 mm.
 17. The side rail ofclaim 1, wherein the region of the first type is constructed as acoupling location for coupling additional components to the side rail.18. The side rail of claim 1, wherein the region of the first type isformed in regions of the side rail which are subject to strongdeformations in a crash or which are configured to dissipate crashenergy through deformations.
 19. The side rail of claim 1, wherein theregion of the first type has an increased wall thickness in relation tothe region of the second type.
 20. The side rail of claim 1, furthercomprising a passage or an edge, or both, in the region of the firsttype after hot-forming.
 21. The side rail of claim 1, wherein the regionof the first type has a stretchability A50 between 10% and 30%.
 22. Theside rail of claim 1, wherein the region of the first type has astretchability A50 between 12% and 20%.
 23. The side rail of claim 1,wherein the region of the first type has a stretchability A50 between12% and 16%.
 24. The side rail of claim 1, wherein the region of thefirst type has a stretchability A50 between 14% and 16%.
 25. The siderail of claim 1, wherein the region of the first type has a tensilestrength between 500 and 1000 N/mm².
 26. The side rail of claim 1,wherein the region of the first type has a tensile strength between 550and 800 N/mm².
 27. The side rail of claim 1, wherein a yield strength ora tensile strength decreases or increases in the transition region witha gradient of more than 100 N/mm² per 10 mm.
 28. The side rail of claim1, wherein a yield strength or a tensile strength decreases or increasesin the transition region with a gradient of more than 200 N/mm² per 10mm.
 29. The side rail of claim 1, wherein a yield strength or a tensilestrength decreases or increases in the transition region with a gradientof more than 400 N/mm² per 10 mm.
 30. The side rail of claim 1, whereinthe region of the second type has a strength of more than 1000 N/mm².31. The side rail of claim 1, wherein the region of the second type hasa strength of more than 1200 N/mm².
 32. The side rail of claim 1,wherein the region of the second type has a strength of more than 1400N/mm².
 33. The side rail of claim 1, wherein region of the first typehas a yield strength between 200 and 800 N/mm².
 34. The side rail ofclaim 1, wherein region of the first type has a yield strength between250 and 600 N/mm².
 35. The side rail of claim 1, wherein region of thefirst type has a yield strength between 250 and 500 N/mm².
 36. The siderail of claim 1, wherein region of the first type has a yield strengthbetween 300 and 500 N/mm².
 37. The side rail of claim 1, wherein theside rail is manufactured from a Tailor Welded Blank or a Tailor RolledBlank.
 38. A method for producing a hot-formed and press-hardened siderail having at least two regions of different hardness, the methodcomprising the steps of: providing a hardenable metal plate orsemi-finished product and heating the hardenable metal plate orsemi-finished product to at least an austenizing temperature,intermediately cooling a region of a first type of the metal plate orsemi-finished product with a cooldown speed selected to be greater thana lower critical cooldown speed of a material of the metal plate orsemi-finished product, and hot-forming and press-hardening the metalplate or semi-finished product in a press-hardening tool to form theside rail.
 39. The method of claim 38, wherein a region of a second typeis held above the austenizing temperature until the region of a secondtype is transported into the press-hardening tool.
 40. The method ofclaim 38, wherein the cooldown speed during intermediate cooling of theregion of the first type is selected such that a bainitic structure isobtained.
 41. The method of claim 40, wherein the region of the firsttype is cooled to a cooling temperature between 600 and 400° C.
 42. Themethod of claim 41, wherein the region of the first type is cooled to acooling temperature of about 500° C.
 43. The method of claim 40, whereinthe region of the first type is held at the cooling temperature for apredetermined time.
 44. The method of claim 40, wherein the region ofthe first type is held at the cooling temperature isothermally.
 45. Themethod of claim 38, further comprising the step of quenching the regionof the first type in the press-hardening tool from a bainitic structuretransformation stage, whereby a mixed structure of martensite andbainite, or a mixed structure of martensite, bainite and at least one offerrite and perlite, is adjusted in the region of the first type. 46.The method of claim 38, further comprising the step of holding theregion of the first type isothermally so as to form a substantially purebainitic structure by press-hardening.
 47. The method of claim 38,wherein the cooldown speed of the intermediate cooling is selected to begreater than an upper critical cooling-down speed.
 48. The method ofclaim 38, wherein the intermediate cooling of the region of the firsttype is performed in the press-hardening tool.
 49. The method of claim48, wherein the intermediate cooling of the region of the first type isperformed by using cooling plates arranged in the press-hardening tool.50. The method of claim 38, wherein the metal plate is pre-formed into asemi-finished product while cold before being heated to at least theaustenizing temperature.